This invention relates to an optical system for providing beam collimation or beam focusing from a multi-emitter or broad emitter semiconductor laser having a phase locked radiation pattern emitting from a common p-n planar junction.
In conventional semiconductor lasers, the beam emitted from a facet of the laser is usually focused to a small spot in order to meet the needs of various applications, such as optical disk storage and laser printing, by means of reimaging the laser near field pattern to the desired image plane. Examples of optical systems designed for beam forming, focusing or shaping are disclosed in U.S. Pat. Nos. 4,203,652; 4,253,735 and 4,323,297.
As indicated in these patents, semiconductor lasers possess different points of origin of divergence (also referred to as beam waist positions) as well as angles of divergence for radiation emitted from the laser facet in directions orthogonal to each other, i.e., in a vertical emission direction which is perpendicular to the p-n planar junction and in a lateral emission direction which is parallel to and along the p-n planar junction. The points of origin of divergence also vary relative to different kinds of laser geometry as illustrated in FIGS. 1 and 2.
In FIG. 1, the radiation beam characteristics for an index guided laser are illustrated. An index guided laser depends on differences in the index of refraction of materials due to the structural geometry of the laser, e.g., nonplanar layers, channels, mesas, etc., to guide the propagating radiation. As illustrated in FIGS. 1A and 1B, radiation in both the vertical and lateral emission direction of laser 10 has a point of origin of divergence or beam waist position 11 at the laser facet 14. This beam waist position 11 is called the near field of the laser. In side view of FIG. 1A, radiation in the vertical emission direction of laser 10 has a point of origin of divergence or beam waist position 11 at facet 14 at the point of emission from the laser optical cavity as represented by by the p-n junction 15. In the top view of FIG. 1B, radiation in the lateral emission direction of laser 10 has a point of origin of divergence or beam waist position 11 also at facet 14 at the point of emission from the laser optical cavity as represented by line 17, which line is illustrative of the laser stripe geometry. Spherical lens 16 collimates beam 12 from laser 10 and spherical lens 18 focuses beam 12 to a spot 20 onto image plane 22. Because the beam waists 11 in both the vertical and lateral directions lie at facet surface 14 in an index guided laser, the near field pattern of the laser can be imaged into a diffraction limited spot 20 at the image plane 22.
A more difficult situation is present in gain guided lasers, illustrated in FIG. 2 where the radiation beam characteristics are illustrated for such a laser. A gain guided laser depends on current dependent differences in the index of refraction of semiconductor materials comprising the structural layers of the laser to guide the propagating radiation. As illustrated in the side view of FIG. 2A, radiation in the vertical emission direction of laser 30 has a point of origin of divergence or beam waist position 31 at the laser facet 34 and at the p-n planar junction 35. Spherical lens 36 collimates beam 32 from laser 30 and spherical lens 38 focuses beam 32 to a spot 40 onto image plane 42. In the top view of FIG. 2B, however, radiation in the lateral emission direction of laser 30 has a point of origin of divergence or beam waist position well within laser 30, e.g., 10 to 40 .mu.m behind facet 34, as indicated at 39. The line 37 is illustrative of the laser stripe geometry and the position of the optical cavity of the laser. Because of this factor, the image plane for the beam in the lateral emission direction will not be in the same plane as the vertical emission direction if lens 37 is omitted. As a result, lens 36 will not be at the proper focal point. With the addition of the convex collimating lens 37, however, the near field pattern of the beam 32 can be collimated for focus by lens 38 to a spot 40 onto image plane 42.
Recently, there has been increased interest in phase locked lasers due to the increased power output provided by a laser over a single filament laser. An early illustration of such a laser is U.S. Pat. No. 3,701,044 to Paoli et al. Paoli et al discloses the optical coupling of adjacent stripe geometry lasers from which a spatially phased locked coherent beam is produced due to the overlapping of the propagating optical wave from one laser optical cavity into an adjacent cavity. A later multi-emitter semiconductor laser is disclosed in the article of D. R. Scifres, R. D. Burnham and W. Streifer entitled "Phase-Locked Semioconductor Laser Array", Applied Physics Letters, Vol. 32(12), pp. 1015-1017, Dec. 15, 1978. A still further improved monolithic multi-emitter laser device is disclosed in U.S. Pat. No. 4,255,717 to Scifres et al. The power output of a recent multi-emitter semiconductor laser has been reported recently by Scifres et al at the Optical Fiber Conference in April, 1982, entitled, "Coupled Multiple Stripe Quantum Well Injection Lasers Emitting 400 mW CW". An optically coupled multiple stripe quantum well injection laser was disclosed which emitted up to a total of 400 mW of continuous output power with over 140 mW CW radiation being coupled into an optical fiber. At this writing, these CW power levels have well exceeded 500 mW. What is needed now is an optical system to focus these large power outputs from muti-emitter or braod emitter lasers to a collimated beam or to a focused spot for various applications, previously mentioned. The conventional optical systems for index and gain guided lasers, geometrically illustrated in FIGS. 1 and 2, are not sufficient. This is illustrated in FIG. 3 showing a top view of a multi-emitter index guided phased locked laser 50. The three stripe contact geometry 55.1, 55.2 and 55.3 illustrates the emission of three beams 52.1, 52.2 and 52.3 from points of origin of divergence or beam waist position at the laser facet 54. Using the conventional optional system of FIG. 1 comprising a lens system 56 to collimate the beams and a lens system 58 to focus the beams to the image plane 62 will provide three spatially focused spots 60.1, 60.2 and 60.3. The same multi-spot imaging would be encountered when using the conventional optical system of FIG. 2 with a gain guided laser.
What is needed in this art is an optical system that makes it possible to combine the output power of the multi-emission or broad emission from such a laser to provide a single high power coherent and collimated beam which may be further focused to a single high power spot.